This program is concerned with circadian rhythms in the old old (80+) and the effect of such rhythms on health and well-being. In particular, it is concerned with sleep, thermoregulation, daytime functioning and phase shift tolerance. There are three projects using human subjects, and one using and animal (rat) model. There are two cores, one (Core A) comprising the (human) sleep and temporal isolation laboratories (and their associated hardware and personnel), the other (Core B) comprising an administrative, biostatistics and subject recruitment core. Project 1 (Reynolds/Jennings) will be a laboratory study of the sleep, sleep deprivation tolerance, and thermoregulatory competence of the old old, comparing them with younger controls. Project 2 (Monk/Jarrett) will comprise a detailed field and laboratory study of the circadian rhythms of the old old, using an "unmasking" technique to isolate endogenous circadian components and to comapre "good" versus "poor" sleepers, and "young" (20-29y) versus old old subjects. Project 3 (Monk) will be a laboratory study of phase shift tolereance in the old old also studying the effects of individual differences and comparing the phase-shift response of old old subjects to that of middle-aged men studied in earlier experiments by the PI. Project 4 (Kittrell) will use the rat model to explore in depth the aging circadian system in its relationship to temperature rhythms, thermoregulatory competence and variables such as behavior and drinking patterns. The approach taken by the program is thus a multi-disciplined one, involving integrated field and human laboratory studies which are related directly to parallel laboratory studies using the rat model. Its overall aim is to enhance our understanding of cyclic regulatory mechanisms in the old old, and thus to ameliorate the disorders of sleep, daytime functioning and thermoregulation that tend to afflict this age group.